1. Field of the Invention
The present invention relates to a cubic boron nitride sintered body and a method of preparing the same, and more particularly, it relates to a cubic boron nitride sintered body obtained by directly converting low-pressure phase boron nitride and simultaneously sintering the same, and a method of preparing such a cubic boron nitride sintered body.
2. Description of the Background Art
A sintered body of cubic boron nitride (hereinafter referred to as cBN), which has the next highest hardness after that of diamond and does not react with any ferrous material, is generally applied to a cutting tool for a ferrous material.
Such a sintered body of cBN applied to a cutting tool is generally prepared by sintering cBN powder with a binder consisting of TiN, TiC, Co or the like under a superhigh pressure. The cBN sintered body obtained in this manner contains about 10 to 40 percent by volume of impurity. This impurity is TiN, TiC, Co or the like forming the binder. This impurity causes reduction of strength, heat resistance and heat dissipativity of the cBN sintered body. Particularly in case of cutting a ferrous material with such a cutting tool at a high speed, its cutting edge is readily chipped or cracked to reduce the life of the cutting tool.
A method of preparing a cBN sintered body without a binder is known for solving this problem. This method is adapted to react and sinter a raw material of hexagonal boron nitride (hereinafter referred to as hBN) with a catalyst such as magnesium boronitride, for preparing a cBN sintered body. The sintered body obtained in this manner has high heat conductivity of 6 to 7 W/cm.multidot..degree. C. since the amount of impurity contained therein is small and cBN grains are strongly bonded to each other therein. Therefore, this sintered body is applied to a heat sink material or a TAB (tape automated bonding) tool. However, the catalyst such as magnesium boronitride remains in this sinteread body. When heated, therefore, fine cracks are readily caused in the cBN sintered body due to thermal expansion difference between the catalyst and cBN. Therefore, this sintered body, having a low heat-resistance temperature of about 700.degree. C., cannot be applied to a cutting tool. Further, the sintered body are formed by large cBN grains of about 10 .mu.M in diameter, and hence its strength is reduced although the stlme has a high heat transfer coefficient. When this sintered body is applied to a cutting edge of a cutting tool, therefore, it is difficult to intermittently cut a workpiece with a high load.
On the other hand, a method of directly converting hBN to a cBN sintered body under a superhigh temperature and a suparhigh pressure with no binder or catalyst is also known. For example, each of Japanese Patent Laying-Open Nos. 47-34099 (1972) and 3-159964 (1991) discloses a method of keeping hBN at a pressure of 7 GPa and a temperature of at least 2100.degree. C. for obtaining a cBN sintered body. In case of industrial production, however, this method is disadvantageous in cost or productivity. In this method, further, large cBN crystals are readily grown due to the sintering under a high temperature, Thus, the strength of the sintered body is so reduced that its cutting edge is chipped when applied to a cutting tool. Further, hBN employed in this method is readily oriented in the &lt;111&gt; direction, and hence the cBN sintered body prepared from thin material is also readily oriented in the &lt;111&gt; direction. When the cBN sintered body obtained by this method is applied to a cutting tool, therefore, layer cracking or separation la diaadvantageously caused.
On the other hand, each of Japanese Patent Laying-Open Nos. 54-33510 (1979) and 8-47801 (1996) discloses a method of preparing a cBN sintered body from pyrolytic boron nitride (hereinafter referred to as pBN) with no binder or catalyst. However, this method is disadvantageous in cost or productivity in case of industrial production. In this method, further, pBN employed as the raw material has an extremely high cost, compressed hBN readily remains in the cBN sintered body, and layer cracking or separation is readily caused due to strong orientation of the cBN sintered body.
Japanese Patent Publication No. 49-27518 (1974) discloses a method of preparing a cBN sintered body with no binder or catalyst under a lower temperature condition by employing hexagonal boron nitride having a mean grain diameter of not more than 3 .mu.m and keeping this raw material at a pressure of 6 GPa and a temperature of 1100.degree. C. In this method, however, the raw material of hexagonal boron nitride consists of fine powder and contains several percent of a boron oxide impurity or adsorbed gas. Thus, sintering insufficiently progresses due to action of the impurity or adsorbed gas. Further, the cBN sintered body containing an oxide of boron is so reduced in hardness, strength and heat resistance that the same cannot be applied to a cutting tool.